Investigation on Characteristic of Tritium Oxidation by Natural Soils

A passive catalytic reactor without heating is required to enhance the safety of a fusion facility. A precious metal catalyst without heating is not suitable to oxidize tritium under conditions of low hydrogen concentration and room temperature. In addition, under a moisture condition, tritium oxidation of a precious metal catalyst drops drastically since moisture adsorbs active sites on the surface of the catalyst. Hence, as a method of tritium oxidation under a moisture condition at room temperature, we have focused on bacterial oxidation of tritium by hydrogen-oxidizing bacteria in natural soil to realize a passive reactor. In this study, we investigated the effect of hydrogen concentration on tritium oxidation by hydrogen-oxidizing bacteria in natural soils to understand the characteristic of tritium oxidation by hydrogen-oxidizing bacteria from the viewpoint ofengineering. In our experiment, efficiency of tritium oxidation by a natural soil was obtained at room temperature in the range of hydrogen concentration from 0.5 to 10000 parts per million (ppm) under a moisture condition. The efficiency of tritium oxidation was the highest at a hydrogen concentration of 0.5 ppm, which equals the value of the hydrogen concentration in air. Our results show that hydrogen-oxidizing bacteria could efficiently oxidize tritium with a low concentration of hydrogen, at room temperature, with high moisture. This showed a tendency opposite to a metal catalyst. A bioreactor using hydrogen-oxidizing bacteria complemented a conventional catalytic reactor using a precious metal catalyst since hydrogen-oxidizing bacteria could oxidize tritium efficiently with a low concentration of hydrogen, at room temperature, with high moisture

Evaluation of Thermal Profile in Catalytic Reactor by Exothermic Hydrocarbon Feed into Detritiation System

Air detritiation system of a fusion facility consists of catalyst reactors for tritium oxidation and a system to remove tritiated vapor from air. In an event of fire in the facility, the impact of gaseous impurities produced by polymeric materials on catalytic oxidation of tritium is one of the points to be evaluated. A point is the impact of reaction heat since gaseous impurities such as hydrocarbons are probable to combust in the catalyst reactors and heat of reactions is large. The distributions of temperature and concentration in a catalytic reactor is numerically evaluated using the thermophysical and chemical properties. The numerical analysis showed that the temperature in a catalytic reactor rose to 1173K by combustion of ethylene and hydrogen when air containing 1% hydrogen and 1% ethylene was fed in a reactor heated initially at 473K. Since the increase in temperature is large, additional technical consideration will be surfaced for design such as selection of material for a reactor, increase in tritium permeation and impact of excessively heated air on equipment located in the downstream. The heat removal mechanism needs to be designed for catalytic reactors.14th International Symposium on Fusion Nuclear Technology (ISFNT-14

Development of precious metal catalyst for tokamak exhaust processing

It is necessary to establish the closed deuterium-tritium fuel cycle in the JA DEMO facility for the purpose of fuel recycling. Tokamak exhaust processing system (TEP) is the key system where hydrogen isotopes and other impurity gases are separated, then impurity gases are decomposed into hydrogen isotopes and chemical compositions without hydrogen isotopes in their chemical forms. The main impurity gas to be processed in TEP is a tritiated hydrocarbon. There are some techniques to process tritiated hydrocarbons. Among existing techniques, chemical processing with precious metal catalyst is a practical technique and precious metal catalyst for TEP needs more consideration to improvement. Among tritiated hydrocarbons, tritiated methane is hard to decompose and temperature of catalyst for decomposition of tritiated methane is 500oC or above with a commercial precious metal catalyst. QST is developing precious metal catalyst for TEP to decompose tritiated methane effectively at a lower temperature. We found that the activity of catalyst for decomposition of methane depends on some parameters such as a kind of precious metal, particle size of precious metal on catalyst surface, pore size of catalyst carrier and so on. Our experimental result revealed that methane can be decomposed effectively by selecting the particle size of precious metal and pore size of catalyst carrier suitable for methane decomposition. In addition, the application of hydrophobization technique to a catalyst for TEP improved the efficiency of methane decomposition much more at a lower temperature.3rd Asia Pacific Symposium on Tritium Science (APSOT-3

Evaluation of thermal profile in catalytic reactor by exothermic hydrocarbon feed into detritiation system

Air detritiation system of a fusion facility consists of catalyst reactors for tritium oxidation and a system to remove tritiated vapor from air. In the event of fire in the facility, the impact of gaseous impurities produced by polymeric materials on catalytic oxidation of tritium is one of the evaluation items. The point is the impact of reaction heat needs to be detected since gaseous impurities such as hydrocarbons are likely to combust in the catalyst reactors as well as heat generation by the combustion reaction is large. The distributions of temperature and concentration in a catalytic reactor are numerically evaluated using the thermophysical and chemical properties. The numerical analysis showed that the temperature in a catalytic reactor rose to 1173 K by the combustion of ethylene and hydrogen when air containing 1% concentration of hydrogen and 1% concentration of ethylene was fed into the reactor heated initially at 473 K. Since the increase in temperature is large, further technical consideration will emerge for designing such as the selection of material for a reactor, the increase in tritium permeation, and the impact of excessively heated air on equipment located at the downstream. The heat removal mechanism needs to be designed for catalytic reactors

Decomposition of methane with various precious metal catalysts for the tokamak exhaust processing

The Tokamak exhaust processing system (TEP) in the deuterium-tritium fuel cycle for DEMO is the key system where hydrogen isotopes and impurity gases are separated. Then the impurity gases are decomposed into hydrogen isotopes and other chemical species. The major impurity gas to be processed in TEP is tritiated hydrocarbons. Among tritiated hydrocarbons, tritiated methane is hard to be decomposed directly since the temperature of catalyst for thermal decomposition is 500oC or above with conventional precious metal supported catalysts such as Pt, Pd, and Rh. The effect of constitutive elements of catalysts on methane decomposition was investigated for a catalyst that could decompose methane efficiently at low temperatures. The investigation showed that the activity of the catalyst for methane decomposition depends on some constitutive elements such as precious metal species, particle sizes of metals, pore sizes of catalyst supports. The experimental results indicated that methane could be decomposed effectively by optimizing precious metal species and the particle sizes of precious metals. Especially the particle size of precious metals had a great influence on methane decomposition efficiency. It was notable that the decomposition efficiency rose 5 times by enlarging a particle size from 12nm to 23nm